1. Field of the Invention
The present invention generally relates to a distortion compensating device, and more particularly, to a distortion compensating device in a transmitter or a receiver which can compensate for nonlinear distortion, the nonlinear distortion being caused by a digital signal being distorted in the path from modulation with a matting in the transmitter to demodulation in the receiver.
2. Description of the Related Art
In a digital multiple radio communication, linear modulation such as phase-shift-keying (PSK) modulation and quadrature-amplitude modulation (QAM) is desired to be used for an effective use of frequency. In terrestrial radio communication, a 256QAM system is currently in practical use. In digital mobile communication systems in which a nonlinear modulation is applicable for portability of radio equipment, 4PSK modulation and 16QAM are currently used. To obtain good transmission performance by using such linear modulation systems, it is necessary to produce a high-power transmission signal.
However, when the power of the transmission signal is increased in conventional multiple communication equipment using linear modulation, a nonlinear range in a transmitting-linear amplifier is used. As a result, the transmission signal is distorted and the transmission performance is degraded. Maximum transmission power is determined by a tradeoff with the distortion. Therefore, to obtain a superior transmission performance, a high-efficiency and high-power linear amplifier is required.
On the other hand, in the receiver, to improve carrier to noise ratio (C/N) performance, an RF amplifier having a large gain is commonly used in the front end. However, a high-level signal is applied to the RF amplifier, the RF amplifier is saturated and the received signal is distorted. Therefore, even though the high-level signal is received, the received signal needs to be reduced by an attenuator, etc., and, thus, the high C/N obtained with the high level signal is degraded. Such attenuation of the high-level signal is performed because sufficient transmission performance for the system can be obtained even if the high-level signal is slightly attenuated. However, in the future, for large capacity data transmission, a much better transmutation performance is desired.
At present, several methods of compensating the distortion due to the nonlinear characteristics of the transmitting amplifier are proposed. FIG. 1 shows a block diagram of an example of a transmitter having a conventional distortion compensating function. The transmitter comprises a mapping-and-waveform-shaping circuit 10, a quadrature modulator 20, a transmission stage 30, and an antenna 40. The transmitter is constructed with a quadrature modulation technique, in which a baseband digital signal is divided into an I-channel signal and a Q-channel signal, and the two signals modulate 90xc2x0-phase-shifted carrier signals to produce a modulated signal by combining the two modulated carrier signals. In the quadrature modulation technique, phase and amplitude of the baseband signal is mapped on a quadrature plane referred to a signal space diagram in which the I axis and the Q axis indicate the carrier signal.
In the mapping-and-waveform-shaping circuit 10, the baseband signals which are divided into the I-channel signal and the Q-channel signal are, respectively, converted to signals for being mapped on the signal space diagram constructed with the I axis and the Q axis, and modulating signals are produced through a roll-off filter. In the quadrature modulator 20, carrier signals Lo are respectively modulated by the I-channel and Q-channel signals, and are summed with each other to produce the modulated signal. In the transmission stage 30, the modulated signal is converted to a signal at the RF frequency, and is amplified by the transmitting amplifier to be transmitted through the antenna 40.
FIG. 2 shows a signal space diagram indicating a relationship between the baseband signal and the carrier signal in 16QAM. For example, in 16QAM, to map the I-channel and the Q-channel signals to one of 16 signal points D1 to D16 (hereinafter, referred to as specified signal points) arranged on the quadrature signal space diagram, the I-channel and Q-channel signals are respectively converted to signals, each having 4 levels. In general, by equalizing each intervals between the 4 levels, the 16 signal points are uniformly arranged on the signal space diagram.
In the receiver side, these signal points are discriminated by threshold lines (represented by dashed lines in FIG. 2) drawn among the signal points in the signal space diagram. The conventional threshold lines are drawn at the center between each two signal points taking into account noise which influences each signal point in the same manner. However, by the strong amplification in the transmission stage 30, the uniformly mapped signal points may be distorted, for example, as shown by signal points X1 to X4 in a first quadrant of FIG. 2. In a more specific case, some distorted signal points may fluctuate to another threshold area such as the signal point X3. As a result, the signal point X3 is discriminated to a wrong signal point as an error. To prevent such fault discrimination, there is a conventional mapping method in which opposite characteristics of the distortion characteristics of the amplifier in the transmission stage 30 are previously set with the mapping circuit 10. By using this method, at the output of the amplifier in the transmission stage 30, a signal point arrangement substantially similar to the specified signal point arrangement may be obtained.
FIG. 3 shows a block diagram of another example of a transmitter having a conventional distortion compensating function. The transmitter comprises a mapping-and-waveform-shaping circuit 11, a quadrature modulator 21, a transmission stage 31 and an antenna 41. Each circuit has the same function as that of the circuit in the transmitter shown in FIG. 1. In the transmitter, according to the shape of the baseband signal passed through the roll-off filter, an operational point of the transmitting amplifier in the transmission stage 31 is changed to extend the dynamic range of the transmitting amplifier. Since the carrier signal is modulated by the baseband signal passed through the roll-off filter, the level of the modulated signal is determined by the amplitude of the baseband signal from the roll-off filter. For example, the level of the modulated signal which is modulated by the baseband signal close to the origin in the signal space diagram is low, and the level of the modulated signal which is modulated by the baseband signal at the surrounding signal points is high. A level change of the modulated signal can be known before transmitting the modulated signal because the baseband signal to be transmitted is known. Therefore, by changing the operational point of the transmitting amplifier according to the level of the modulated signal, dynamic range of the amplifier may be increased.
However, the following problems occur in the above-mentioned conventional distortion compensating devices.
In the distortion-compensating device shown in FIG. 1, when the distortion characteristics due to the nonlinearity of the transmitting amplifier in the transmission stage 30 changes, the proper distortion compensation may not be performed with the previously-set opposite characteristics in the mapping-and-waveform-shaping circuit 10. The case where the distortion characteristics of the transmitting amplifier change may be due to a change of the input level applied to the transmitting amplifier, an exchange of the transmitting amplifier, and a temperature increase and aging of the transmitting amplifier. Therefore, the opposite characteristics need to be adapted to such changes. This adaption requires the hardware to be complex. More specifically, to deal with the exchange of the transmitting amplifier, the transmission stage 30 in the RF circuit and the mapping-and-waveform-shaping circuit in the baseband circuit need to be integrated. This may cause complexity of the configuration of the distortion compensating device.
In the distortion compensating device shown in FIG. 3, the waveform-shaping circuit needs to be constructed with a digital signal processing, and an amplifier whose operational point is controllable is required for the transmission stage 31. Therefore, the configuration of the distortion compensating device becomes complex.
Furthermore, in the distortion compensating devices shown in FIGS. 1 and 3, only distortion occurring in the transmitter side is compensated, the distortion occurring in the receiving amplifier in the receiver side cannot be compensated.
It is an object of this invention to provide a distortion compensating device. The distortion compensating device may compensate signal distortion which occurs in a path from modulation in a transmitter to demodulation in a receiver regardless of characteristics of the transmitting and receiving amplifiers. The distortion compensating device is easily constructed with a digital signal processing technique, and is capable of being integrated to a 1-chip IC. Further, by using the distortion compensating device, it is possible to increase transmission power in the transmitter without attenuating a receiving level in the receiver. Therefore, a high system gain in the path from the transmitter to the receiver may be achieved. This permits the disadvantages described above to be eliminated.
The object described above is achieved by a distortion compensating device in a receiver for compensating distortion which is added to a signal in a transmission path, the signal being generated in a multi-level modulator of a transmitter in which a digital signal is mapped to one of plural specified signal points in a signal space diagram, the device comprising: a signal-point-position averaging circuit for producing an average signal point for each of a plurality of distributions of received signal points in the signal space diagram; and a discrimination circuit for discriminating the average signal point closest to a received signal point from other average signal points in the signal space diagram.
According to the above distortion compensating device, from a plurality of average signal points produced by the signal-point-position averaging circuit in the receiver, the average signal point closest to the received signal point is discriminated. Therefore, when the received signal point deviates due to the nonlinear distortion of the amplifier, a fault decision for the received signal based on the conventional threshold lines may be decreased.
The object described above is also achieved by a distortion compensating device in a receiver for compensating distortion which is added to a signal in a transmission path, the signal being generated in a multi-level modulator of a transmitter in which a digital signal is mapped to one of plural specified signal points in a signal space diagram, the device comprising: a signal-point-position averaging circuit for producing an average signal point for each of a plurality of distributions of received signal points in the signal space diagram; and a discrimination circuit for defining threshold lines in a checkered pattern between the average signal points in the signal space diagram, and for discriminating a received signal point by the threshold lines.
According to the above distortion compensating device, new threshold lines are defined between the average signal points produced from the signal-point-position averaging circuit. The received signal point is discriminated according to the new threshold lines. Therefore, when the received signal point deviates due to the nonlinear distortion of the amplifier, the fault decision for the received signal based on the conventional threshold lines may be decreased.
The object described above is also achieved by a distortion compensating device in a receiver for compensating distortion which is added to a signal in a transmission path, the signal being generated in a multi-level modulator of a transmitter in which a digital signal is mapped to one of plural specified signal points in a signal space diagram, the device comprising: a signal-point-position averaging circuit for producing an average signal point for each of a plurality of distributions of received signal points in the signal space diagram; and a discrimination circuit for shifting a received signal point by a distance based on a difference between the specified signal point and the corresponding average signal point in an opposite direction of that from the specified signal point and the corresponding average signal point, and for discriminating a shifted received signal point by threshold lines defined by the specified signal points.
According to the above distortion compensating device, the received signal point is shifted by the distance based on the difference between the specified signal point and the corresponding average signal point in the opposite direction of that from the specified signal point and the corresponding average signal point, and the shifted received signal point is discriminated by the conventional threshold lines defined by the specified signal points. Therefore, by shifting the received signal point, the distortion added to the received signal may be compensated.
The object described above is also achieved by a distortion compensating device in a receiver for compensating distortion which is added to a signal in a transmission path, the signal being generated in a multi-level modulator of a transmitter in which a digital signal is mapped to one of plural specified signal points in a signal space diagram, the device comprising: an error-signal generation circuit for generating an error signal indicating a relationship between a corrected received signal point and the corresponding specified signal point in the signal space diagram; an average-error-signal calculating circuit for producing an average error signal by accumulating the latest error signal from the error-signal generation circuit for each received signal supply; a correction circuit for producing the corrected received signal by subtracting the average error signal from a received signal; and a discrimination circuit for discriminating the corrected received signal from the correction circuit.
According to the above distortion compensating device, the received signal point is corrected in the direction of the specified signal point based on an average value of the error signal indicating the relationship between the corrected received signal point and the corresponding specified signal point, and the corrected received signal point is discriminated by the conventional threshold. Therefore, by correcting the received signal point, the distortion added to the received signal may be compensated.
The object described above is also achieved by a distortion compensating device in a receiver for compensating distortion which is added to a signal in a transmission path, the signal being generated in a multi-level modulator of a transmitter in which a digital signal is mapped to one of plural specified signal points in a signal space diagram, the device comprising: a distortion-compensated signal calculating circuit for producing a distortion-compensated signal S which is calculated by an equation:
S=Vxe2x88x92xcex12V2xe2x88x92xcex13V3. . .,
where V is a received signal, and xcex1n is a given factor.
According to the above distortion compensating device, as shown in the equation, a distortion component is subtracted from the received signal, and the received signal, after the subtraction, is discriminated by the conventional threshold. Therefore, by the above calculation, the distortion added to the received signal may be compensated.
The object described above is also achieved by a distortion compensating device in a receiver for compensating distortion which is added to a signal in a transmission path, the signal bring generated in a multi-level modulator of a transmitter in which a digital signal is mapped to one of plural specified signal points in a signal space diagram, the device comprising: a discrimination circuit for discriminating a received signal according to threshold lines partitioning the signal space diagram into discrimination areas, the threshold lines being defined so that outer side discrimination areas can be wider than inner side discrimination areas.
According to the above distortion compensating device, the threshold lines partitioning the signal space diagram into the plurality of discrimination areas are defined so that the outer side discrimination areas can be wider than the inner side discrimination areas, and the received signal is discriminated by the threshold. More specifically, when the received signal level in the receiver is high, the above type threshold is selected. Therefore, when the received signal point deviates due to the nonlinear distortion of the amplifier, the fault decision for the received signal based on the conventional threshold lines may be decreased. Further, since it is possible to increase transmission power without attenuating the received signal level, this enables a system gain to increase.
The object described above is also achieved by a distortion compensating device in a transmitter for compensating distortion which is to be added to a signal in a transmission path, the signal being generated in a multi-level modulator of a transmitter in which a digital signal is mapped to one of plural specified signal points in a signal space diagram, the device comprising: a variable mapping circuit for changing a mapping pattern of the specified signal points according to a level of a received signal in a receiver.
According to the above distortion compensating device, in modulation of the transmitter, the digital signal is mapped to an outer-shifted signal point when the received signal level in the receiver is high. Therefore, when the signal is distorted by nonlinear characteristics due to the high level of the signal, even if the received signal point in the receiver is deviated by the distortion, a possibility of the fault decision may be decreased. Accordingly, since it is possible to increase transmission power without attenuating the received signal level, this enables the system gain to increase.
The above-mentioned distortion-compensating devices are respectively constructed independent of the characteristics of the transmitting amplifier and the receiving amplifier. Therefore, the device may compensate distortion added in the transmission path from the modulation in the transmitter to the demodulation in the receiver.
Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings.